Hello student as per source you can't cancel your retention form after admission if you wish to cancel it then your admission will also get cancel because its a final process. You can also click on the link mentioned below for more information
http://view.mahacet.org/mahacet/admin/news_document/NEET%20Health_Sciences_Brochure%202019%20-Final_compressed.pdf

Feel free to comment if you have any doubt
Good luck